The resonator fiber optic gyro (RFOG) shows promise of meeting challenging demands of a large number of inertial guidance applications. To meet cost and size requirements, much of the RFOG laser source optical functions may be employed with silicon photonics (SiP) chip technology. Many of the optical functions such as waveguides, optical couplers and splitters, intensity modulators, and photodiodes can perform just as well or even better than their discrete optical device counterpart. However, an optical phase modulator is difficult to employ in silicon while meeting even-harmonic distortion requirements because of their inherent nonlinearity. This limitation can maybe overcome with the use of subharmonic modulation, where the inherent even-function nonlinearity is used as advantage to generate phase modulation at the desired frequency while meeting even harmonic distortion requirements. However, using subharmonic modulation introduces new sources of possible errors for the gyro. A gyro error can occur when either the voltage modulation drive or the SiP modulator voltage to phase transfer function has an offset. There is a need for a system and method to detect the relative offset between applied voltage and optical phase and control it to zero.